Optionally include C run-time support

If the C run-time library is compiled and installed on your system, you can now
do 'cabal configure -fc-runtime' to get the following extras:
  + The haskell binding to the C run-time library will be included in the
    PGF library (so you can import it in Haskell applications).
    Documentation on the new modules will be included when you run
    'cabal haddock'.
  + The new command 'pgf-shell', implemented on top of haskell binding to
    the C run-time system.
  + Three new commands in the web API: c-parse, c-linearize and
    c-translate. Their interfaces are similar to the corresponding commands
    without the "c-" prefix, but they should be considered preliminary.

gf.cabal

@@ -1,7 +1,7 @@
 name: gf
 version: 3.5.11-darcs
 
-cabal-version: >= 1.8
+cabal-version: >= 1.10
 build-type: Custom
 license: OtherLicense
 license-file: LICENSE
@@ -57,7 +57,12 @@ flag custom-binary
   Description: Use a customised version of the binary package
   Default:     True
 
+flag c-runtime
+  Description: Include functionality from the C run-time library (which must be installed already)
+  Default:     False
+
 Library
+  default-language:    Haskell2010
   build-depends: base >= 4.2 && <5,
                  array,
                  fst,
@@ -113,7 +118,16 @@ Library
     PGF.VisualizeTree
     PGF.OldBinary
 
+  if flag(c-runtime)
+    exposed-modules:     CId, CRuntimeFFI
+    other-modules:       Gu, PgfLow
+    hs-source-dirs:      src/runtime/haskell-bind
+    build-tools:         hsc2hs
+    extra-libraries:     gu pgf
+    cc-options:          -std=c99
+
 Executable gf
+  default-language:    Haskell2010
   build-depends: gf,
                  base >= 4.2 && <5,
                  array,
@@ -140,6 +154,8 @@ Executable gf
     other-modules: GFServer
     hs-source-dirs: src/server src/server/transfer src/example-based
 
+  if flag(c-runtime)
+    cpp-options: -DC_RUNTIME
 --if flag(new-comp)
 --  cpp-options: -DNEW_COMP
 
@@ -232,6 +248,13 @@ Executable gf
   else
     other-modules: GF.System.NoSignal
 
+executable pgf-shell
+  if !flag(c-runtime)
+    buildable: False
+  main-is:             pgf-shell.hs
+  hs-source-dirs:      src/runtime/haskell-bind/examples
+  build-depends:       gf, base
+  default-language:    Haskell2010
 
 test-suite rgl-tests
   type:           exitcode-stdio-1.0